A strain is a low-level taxonomic rank used at the
intraspecific level (within a species). While the majority of the members of the same species have very high similarity in their genomes, there occasionally is a proportion of the species that has a little different genome from the popular species group. This variability results in different strains on the species.
A harmless Staphylococcus
aureus may have a muted virulence gene. However, either due to mutation of the
gene or recombination with the harmful strain, a harmless Staphylococcus
aureus could easily turn virulent (harmful).
the first answer as blood needs to be oxygenated to be passes round the body then when its deoxgenated it will return to the heart to be pumped around the body again
